Frames used in automated two- or three-wheeled vehicles can be broadly classified as (a) those which use pipe as their primary material, i.e., pipe frames, and (b) those which use pressed steel plates, which are welded together to form a steel plate frame.
The latter type of construction permits automatic welding, which results in high productivity, but for reasons of strength, it is applicable only to light-weight vehicles. By contrast, pipe frame construction provides adequate strength, and this type of construction is widely used for a variety of leisure, sport, competition and touring vehicles.
In the past, cast or forged lugs have been used to connect the various types of pipe material. More recently, in order to produce lighter vehicles and to reduce costs, lugs have been eliminated in many cases in favor of arc welding or, in certain cases, by use of gussets made of steel plate and welding.
However, welded connections create several problems:
(1) If the welders are not highly trained, the welds will contain undercuts, overlaps or other deformities. When this kind of welding defect is present, because of the accumulation of stress upon the welded area, fatigue strength declines, so that further processing is required, such as polishing the weld area flat, shot blasting or shot peening, or heat treatment of the weld area. Obviously, this results in increased production costs.
(2) In order to reduce frame weight, high tensile strength steels have been used. When such steels have been subjected to tempering or annealing, the large input of heat caused by welding produces soft areas which cause the yield point of the frame to decrease. This phenomenon is not confined to high tensile strength steels; the input of heat also causes such defects in normal steels. When this occurs, the entire vehicle frame must be subjected to further processing directly after welding, at which time close attention is required to prevent frame deformation. Such processing requires large scale facilities and increases production steps and hence costs.
(3) Making the connecting lugs for the various pipes used in the frame from a light weight alloy is effective in reducing the proportion of lug weight in the total frame weight. However, because the lug and pipe are made of different materials, welding is impossible.
(4) Even in cases where brazing (a type of welding) is employed, the problems mentioned in item (2) are encountered, since the brazing temperature exceeds the annealing temperature.
(5) Whether or not lugs are used, the heating caused by welding can cause the frame to warp, so that, after welding, such warp must be corrected.